deps: bump bdk to beta-1

[rustaceanrob]

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All Submissions:

• [ ] I've signed all my commits
• [ ] I followed the contribution guidelines
• [ ] I ran cargo fmt and cargo clippy before committing

New Features:

• [ ] I've added tests for the new feature
• [ ] I've added docs for the new feature

Bugfixes:

• [ ] This pull request breaks the existing API
• [ ] I've added tests to reproduce the issue which are now passing
• [ ] I'm linking the issue being fixed by this PR